With current implementations of caller identification, the called party receives the phone number of the phone used by the calling party. This system works when the caller uses a predetermined phone (e.g., their home phone or their cell phone) to place the call. When the call is placed from an unrecognized phone (e.g., public phone, lobby phone, etc.), the phone number from which the call has been placed provides little if any information about the caller.
There are particular situations wherein a person would like to make their private phone number available to the other party for only a limited time, or reserve the ability to block future phone calls from a specific person altogether. For example, a doctor may want to give their phone number to a patient after a surgery and make it available for them until they are no longer in a critical condition. It may be desired to disable this direct access after the patient recovers and enforce the access through normal channels (appointment desk, nurse, etc.). People who are dating may desire to be able to give out their phone number and preserve the ability to block calls from a specific caller regardless of the phone the caller is using to make the call.